Radio frequency antenna system

ABSTRACT

Tactical radio navigation systems provide distance measuring information to an interrogating aircraft in response to pairs of interrogation pulses received at a beacon transponder, the system also provides bearing and identification information. The transponder receives pairs of interrogation pulses which are decoded into a single pulse for operation of a reply transmitter. Bearing, distance and identification determining signals are generated by the reply transmitter and its antennas. A single antenna functions to both receive the interrogation pulses and radiate the position determining signals. To provide the required bearing information, the antenna radiates pulses from a central antenna array that is stationary with respect to a support housing. These pulses are modulated at a 15 Hz frequency by parasitic elements rotating around the stationary central array. Further modulation of the transmitted pulses at a 135 Hz frequency, superimposed on the 15 Hz frequency, is provided by additional parasitic elements also rotating about the central antenna array. Both the low band frequency parasitic elements and the high band frequency parasitic elements have a configuration to provide improved radiation from the antenna above the horizontal. Also to improve the antenna radiation pattern, the 135 Hz parasitic elements are positioned on a rotating drum of a dielectric material, such as fiberglass. To still further improve the amount of radiation energy above the horizontal reference, radio frequency chokes are coupled to the central antenna array in the feed line from the signal transmitter.

Unite States m Pickles et a1.

Primary Examiner-Maynard R. Wilbur Assistant Examiner-S. C. BuczinskiABSTRACT Tactical radio navigation systems provide distance measuringinformation to an interrogating aircraft in response to pairs ofinterrogation pulses received at a beacon transponder, the system alsoprovides bearing and identification information. The transponderreceives pairs of interrogation pulses which are decoded into a singlepulse for operation of a reply transmitter. Bearing, distance andidentification determining signals are generated by the replytransmitter and its antennas. A single antenna functions to both receivethe interrogation pulses and radiate the position determining signals.To provide the required bearing information, the antenna radiates pulsesfrom a central antenna array that is stationary with respect to asupport housing. These pulses are modulated at a 15 Hz frequency byparasitic elements rotating around the stationary central array. Furthermodulation of the transmitted pulses at a 135 Hz frequency, superimposedon the 15 Hz frequency, is provided by additional parasitic elementsalso rotating about the central antenna array. Both the low bandfrequency parasitic elements and the high band frequency parasiticelements have a configuration to provide improved radiation from theantenna above the horizontal. Also to improve the antenna radiationpattern, the 135 Hz parasitic elements are positioned on a rotating drumof a dielectric material, such as fiberglass. To still further improvethe amount of radiation energy above the horizontal reference, radiofrequency chokes are coupled to the central antenna array in the feedline from the signal transmitter.

22 Claims, 9 Drawing Figures I35 HZ PARASITIC ELEMENTS 0x005 CURRENTSHLO, sNoo- 83 SIGNAL o- 1 l RETURN 15H: PARASITIC M T ELE ENS 00 I 15Hz PICKOFF I35 Hz PICKOFF 4) [350 Hz PICKOFF PATENTED 5W EQTiQBEMS 3mm 1w DIODE CURRENT gHLD.GND -H SIGNAL RETURN MAIN RF 88 Q 0 l5 HZ PICKOFF Q0 B5 HZ PICKOFF A L i 0 1350 H2 PICKOFF FiG.

PAIENIEU 5% 3f?@@%@@ k DIOPOLE POWER DIVIDER AND PHASER RF INPUT m l5 HzMODULATION I35 Hz 100 MODULATION RADIO FREQUENCY ANTENNA SYSTEM Thisinvention relates to a radio navigation antenna system and moreparticularly to a radio navigation system with improved radiation energyuptilt above a horizontal reference.

Radio navigation systems, such as TACAN, provide bearing information anddistance information by a beacon station which radiates pulses accordingto a rotating multilobe directional pattern. Each time the low frequency(e.g., Hz) lobe of this patern passes a given reference point, e.g., anorth reference, a reference signal is emitted. Rotation of the antennapattern, in effect, amplitude modulates the pulses to provide anamplitude envelope thereon. The phase information of this envelope withrespect to the reference signal varies at different azimuthal anglesfrom the beacon. In a receiver of an interrogating craft these pulsesare received from the beacon station and the envelope is separated fromthe pulses by a filter and the phase of this envelope is compared withthe phase of the reference signal to give bearing indication. Thedistance data is derived by electronically measuring the net timeelapsing between transmission of an interrogation pulse pair and receiptof a reply pulse pair from the surface beacon. Subtracting theprocessing delay, the resultant time interval is directly proportionalto the line of sight distance between the interrogating aircraft and thebeacon.

Heretofore, distance measuring systems required the use of a high powerbeacon transmitter to generate interrogating transmission at sufficientpower to enable the interrogating aircraft to obtain a position fix.Such high power transmitters are difficult to construct and maintain toreliably provide the requisite information signals. Considering thatmany TACAN systems are portable, this further increases the complexityof the source of transmitted energy.

With regard to the antenna itself, generally speaking, prior antennas ofthe pseudoportable type provided a narrow vertical aperture and hadrelatively poor electrical performance. Further, the weight and powerconsumption of the spin motor and associated speed control was alsofound to be excessive.

Several attempts have been made to eliminate the reradiation reflectionproblem in radio navigation systems. These attempts have employedantennas that used a cardioid pattern of a central radiator with 15cycle reflectors to excite mechanical l35 cycle reflectors. Under theseconditions, the 135 cycle reflectors have not been uniformly illuminatedwith the result in the production of a considerable amount of 120 Hz and150 Hz modulations in addition to the 15 cycle and 135 cyclemodulations. In this type of antenna, the configuration of metal partswithin the confines of the modulating members provides particularsusceptibility to the production of re-radiation signals. The modulatingelements are individually nondirectional and as such reradiate in alldirections and as such some of the radiation returns to the centralradiator. Radiation not along the radius of the central radiator strikesthe metal reflective surface at an oblique angle and the wave reflectedfrom the oblique incidence is no longer in a vertical plane. Since thisenergy comes from the modulating elements it is information energy withthe phase thereof not predictable. Therefore, when a receiving antennaon an interrogating aircraft receives these signals the results areusually quite different'from the correct azimuth phase. A feature ofthis invention is the elimination of the type designs described above.

In accordance with the present invention, an improved radiation patternis accomplished by controlling the environment of the radiating andreflector elements. There is a minimum of oblique metal surfaces withinthe confines of the antenna system which would produce or convertradiation into a plane at angles other than that desired. Thus, theantenna of the present invention provides improved effective versusapparent vertical aperture. A central array of the antenna exhibitsimproved uptilt of the radiation pattern and slope characteristicsacross the usual radio navigation frequency spectrum. Still anotherfeature of the present invention is the control of the sidebandradiation pattern by means of the parasitic reflectors and the mountingthereof on a nonmetallic rotating drum. The nonmetallic underside of therotating drum also reduces radio frequency spillover and reduces theenergy radiated below the horizon of the antenna.

To better understand the invention, a brief discussion of the generalTACAN principles as related to the present radio navigation system willbe given. Tactical air navigation (TACAN) is a radio air navigationsystem of the polar coordinate type which provides an aircraft withdistance measuring information (DME) and bearing information. Usually, ameter in an interrogating aircraft indicates, in nautical miles, thedistance of the aircraft from a surface beacon. Another meter indicatesdirection of flight in degrees of bearing with respect to the geographiclocation of the surface beacon and magnetic north. By knowing thebearing and distance from a Specific geographic point, i.e., the groundstation beacon transponder, the pilot of an aircraft can fix hisposition. An identification signal from the surface beacon enables thepilot to identify which beacon he is receiving information from andtherefore allows him to plot his geographic location.

The distance measuring concept used in TACAN equipment is an outgrowthof radar-ranging techniques, wherein distance is determined by measuringthe round-trip travel time of pulsed radio frequency energy. A returnsignal (echo) of the radiated energy is dependent on natural reflectionsof the radio waves. Radio navigation systems for use with the antenna ofthe present invention are located at specific geographic positions andgenerate artificial replies rather than depending on naturalreflections. The airborne equipment generates timed interrogationpulse-pairs that are received by the surface beacon-transponder systemand decoded. After a SO-us delay, the transponder transmits areplysignal. The round-trip time is then converted to distance from theground facility by the airborne DME equipment. With the ground positionknown and the distance known, the aircraft location can be positioned onthe perimeter of a circle whose radius is equal the measured distance.The timing of the pulses generated is of primaryimportance for thedistance information. To convey bearing information to an interrogatingaircraft, amplitude modulation of the pulses from the ground transponderare employed. Bearing information is produced by a specificdirectional-radiation pattern rotated around a vertical axis. Thissignal, when properly referencedto magnetic north, as explained,identifies the aircraft direction from the ground facility. The magneticnorth bearing information and the distance-data gives a two point fixfor a specific aircraft location.

In a system wherein the present invention is embodied, radio frequencyenergy is fed to a stationary central antenna array of an antennasystem. This central array has no directivity in the horizontal plane.Vertical parasitic elements are rotated around the central array at afixed number of revolutions per second. The distance between the centralarray and the parasitic elements is established to obtain a desiredcardioid radiation pattern. To an aircraft at a specific location, thedistance-data pulses would contain a low frequency amplitude-modulatedsignal due to the rotation of the cardioid radiation pattern. Bearinginformation can be obtained by comparing the low frequency modulatedsignal with a reference frequency signal received from the groundfacility. The phase relationship between the two low frequency signalswill be dependent on the location of the receiving aircraft and thecardioid pattern.

A suitable reference signal pulse transmitted at the same fixed phase ofthe low frequency will serve as well as a complete wave for thereference signal. These signals are sent out when the maximum of therotating cardioid pattern aims due magnetic east, provided the antennais aligned due magnetic north.

For improved accuracy, a group of additional parasitic elements, mounteda fixed number of degrees apart, also rotates around the central antennaarray along with the low frequency elements and further modify thecardioid radiation pattern. Although the cardioid pattern is stillpredominant, it is altered by superimposed ripples. The interrogatingaircraft now receives the low frequency with a higher frequency rippleamplitude modulated on the distance data reference and squitter pulses.To furnish a suitable reference for measuring the phase of the highfrequency component of the envelope wave, a coded high frequencyreference signal pulse is transmitted from the beacon transponder.

In one embodiment of the present invention, a radio frequency antennasystem comprises a housing that includes a drive motor. A centralantenna array of omnidirectional antennas is enclosed in an antenna tubemounted to the support housing to be fixed in position with respectthereto. A plurality of spaced parasitic antenna elements are positionedfrom the central antenna array on a support means coupled to the drivemotor. The drive motor rotates the support means and the parasiticantenna elements around the central antenna array to modulate theelectromagnetic waves radiating therefrom. A series of radio frequencychokes are connected to the central array to provide improved uptilt tothe radiation energy and minimum re-radiation energy.

A more complete understanding of the invention and its advantages willbe apparent from the specification and claims and from the accompanyingdrawings illustrative of the invention.

Referring to the drawings:

FIG. 1 illustrates a portable installation of a radio navigation systememploying the antenna of the present invention;

FIG. 2 is a block diagram of a dual configuration radio navigationsystem employing the antenna of the present invention;

FIG. 3 is a schematic diagram of an antenna system having 15 Hz lowfrequency and Hz high frequency parasitic elements rotating about acentral antenna ary;

FIG. 4 is a free space energy pattern for the central antenna array ofFIG. 3 showing the uptilt radiation above the horizon;

FIG. 5 is a top view of a physical phasing diagram of an antenna patternof the type schematically shown in FIG. 3;

FIG. 6 is a cross-section of an antenna system having a rotating drumsupporting parasitic elements revolving about a central antenna array;

FIG. 7 is an enlarged view of the central antenna array of FIG. 6;

FIG. 8 is an exploded view of the antenna system shown in FIG. 6; and

FIG. 9 shows an alternate embodiment for mounting the parasitic elementsfor improved uptilt radiation.

Referring to FIG. 1, there is shown a typical installation of a radionavigation system wherein an antenna 10 is mounted on a tripod 12 in anarea preferably free from as much ground reflection and clutter aspossible. A control unit 14 is coupled to the antenna 10 and providesfor rotation of the parasitic elements (to be described) in response tosignals from a control/transfer unit 16. Also mounted to the tripod 12is a monitor antenna 18 which connects to the control/transfer unit 16through cabling 20. The monitor antenna 18 provides means for checkingthe operation of the antenna 10. The control transfer unit 16 alsoconnects to a trigger sensor unit 22 as part of the antenna 10.

Interrogation pulses received by the antenna 10 are transmitted to thecontrol/transfer unit 16 for processing in receiver/transmitter units 24and 26 both of which are similar in construction and operation toprovide a dual configuration, standby capability. System monitoringsignals received by the monitor antenna 18 are coupled through thecontrol/transfer unit 16 to monitors 28 and 30. The monitor unitsprovide a continual check on the operation of the system to insurereliability of system operation. An oscilloscope 32 connects to thecontrol/transfer unit 16 to provide visual information on the operationof the system.

Referring to FIG. 2, there is shown a block diagram of the various unitsof FIG. 1 including a block representing the antenna 10. For correctorientation of the antenna with reference to a reference mark, there isincluded as part of the antenna structure a reference target alignmentindex 34. When setting up the antenna for system operation, properorientation is necessary. Within the structure of the'antenna 10 is acentral antenna array 36 that is mounted to a housing 38 and heldstationary with respect thereto.

The central antenna array 36 receives the interrogation pulse pairs frominterrogating aircraft and also radiates distance, bearing andidentification information pulses. The transmitted pulses are modulatedat a first frequency by means of parasitic elements 40 mounted to rotateabout the central array 36. Superimposed on this low frequencymodulation is a higher frequency modulation provided by parasiticelements 42. In the embodiment shown, there are two low frequencyparasitic elements and nine high frequency parasitic elements. Boththese sets of elements are rotatable about the central array 36 by meansof a motor drive 44 mechanically coupled to the supporting structure forthe elements. Also driven by the motor 44 is a light disc 46 forproviding timing information to a reference trigger source 48.

Aircraft interrogation radio frequency signals from the central antennaarray 36 are transmitted to the control/transfer unit 16 by means of acoaxial cable 50. Depending on the position of a transfer switch 52,signals from the antenna are routed to either the receiver/- transmitter24 or the receiver/transmitter 26. In the position shown for thetransfer switch 52, the transmitter 24 processes the interrogation pulsepair received from the transfer unit 16 over a coaxial cable 54. Thetransmitter 26 is then in a standby mode and responds to testinterrogations and produces test replies to a dummy load 56. If thetransmitter 24 fails, the monitors 28 and 30 sense the failure and sendan alarm signal which causes the transfer switch 52 to switch to thestandby transmitter 26. In this sutation, then the transmitter 24becomes the standby unit, the monitors 28 and 36 hold in memory thefailed parameter for maintenance purposes.

Fundamentally, the transmitters 24 and 26 detect and decode weakinterrogations at one frequency and generate higher power replies atanother frequency. Another function of the transmitter units 24 and 26is to produce a Morse code identification message periodically for aninterrogating aircraft to identify the transmitting source of distanceand bearing information received. The receiver section of the receiver/-transmitter 24 and 26 receives and logically recognizes aircraftinterrogation in a function module represented by block 58. This networkis coupled to a duplexor function module represented by block 60.Interrogations arriving at the antenna 36 are applied to a preselectorincluded as part of the duplexor module 60 and those arriving at thecorrect frequencies pass through to be applied to the function module 58which provides output pulses to an encode module represented by block62. The encode module 62 also receives timing pulse information from thereference trigger sensor 48 through the control unit 14 and thecontrol/transfer unit 16. The control unit 14 includes a referencetrigger amplifier 64 for amplification of the timing pulses. Pulsesreceived at the encode module 62 from the module 58 are converted intohigh frequency radio impulses at a transmit frequency 63 MHz away fromthe received frequency. The output pulse train consists of reply pulses,position (squitter) pulses and identify pulses; all output pulses areused to carry the position information. Also generated in the encodemodule 62 are the reference pulses synchronized with the timing pulseinput.

Broadcasted pulse trains from the module 62 are coupled to a transmitmodule represented by block 66 which has an output coupled to theduplexor module 60. Pulse trains at the output of the module 66 includethe distance, bearing and identification information in a timedrelationship with referenced pulse information and identity pulseinformation. These pulses are transmitted to the central antenna array36 of the antenna through the coaxial cables 50 and 54 and the transferswitch 52.

As mentioned, the system is continuously monitored by means of a monitorantenna 18 coupled to the transfer unit 16 through a coaxial cable 68.Signals received from the antenna 18 are processed through a dividercircuit 70 of the control transfer unit 16 and distributed to themonitors 28 and 30 over coaxial cables 72 and 74, respectively. Alsoreceived by the monitors 28 and 30 are trigger signals from thereference trigger amplifier 64. Each of the monitors 28 and 30 includesa test interrogation generator (block 76), a signal analysis and modestepping module (block 78), and a display module (block 80). Asexplained, if the functional receiver/transmitter fails, the monitorssense the failure, store the failure in memory and switch systemoperation to the standby transmitter. Both monitors 28 and 30 are activeat all times and both must sense a failure before a transfer or shutdownoccurs. A shutdown occurs under certain operating conditions or if bothtransmitters 24 and 26 have failed. The monitors 28 and 30 are solelyfor the purpose of system monitoring and testing and do not provide forthe basic function of the system, that is, providing distance, bearingand identification information from the centrla antenna relay 36.

Referring to FIG. 3, the central antenna array 36 is a multielementarray consisting of two half wavelength dipoles 82 and 84 stacked on topof one another. A pulse radio frequency is fed to the terminals 86 and88 of an RF input connector and is routed to the stacked array ofdipoles 82 and 84.

Referring to FIG. 4, the dipoles 82 and 84 are fed through a powerdivider/phaser 94 in such a manner that the radiation pattern is notsymmetrical. The peak of the main lobe 96 is tilted upward above thehorizon. As an example, the uptilt is adjusted to center the free spacemain lobe at approximately +30 above the horizon. The purpose of theuptilt is to reduce the energy that is radiated below the horizon. Thispositive angle assures that the major portion of the radiated energy isconcentrated at a positive angle (i.e., above the horizon). Also, thearray is vertically polarized and has a circular radiation pattern.

An aircraft receiving signals from the antenna 10 will receive a directray and also any reflected rays. If the direct ray and the reflected rayarrived in phase at the receiving point, the signals will add, andconversely if the rays arrive out of phase, they subtract and theresultant signal at the receiving point becomes weaker. These areas arecalled maxima and minima (or nulls). The depth of a null is equal to theenergy of the direct ray minus the energy of the reflected ray. If therays are equal in amplitude and opposite in phase at the receivingpoint, complete cancellation occurs and the resultant signal is zero.Thus, it is important that as much energy as possible be radiated abovethe horizon and energy below the horizon be minimized..lt is the energythat is radiated from the antenna below the horizon which strikes theearth and is re-radiated to produce the null characteristic.

The radio frequency energy emitting from the central antenna array 36has no directivity in the horizontal plane. To convey bearinginformation to a using aircraft, pulses from the central array areamplitude modulated. Referring again to FIG. 3, two split verticalparasitic elements 98 are mounted to rotate around the dipoles 82 and 84at a fixed number of revolutions per second. The distance between thedipoles 82 and 84 and the parasitic elements 98 along with the speed ofrotation of the parasitic elements establishes the modulation frequencyapplied to the pulses from the central antenna array. The parasiticelements 98 thus impose an amplitude modulation component on thebroadcast pulse train from the central antenna array.

The basic cardioid pattern of energy from the antenna is illustrated inFIG. which shows a top view of the radiation pattern. To an aircraft ata specific location, all pulses now contain an amplitude modulatedsignal due to the rotation of the cardioid radiation pattern. Bearinginformation is obtained by comparing the modulated signal with areference signal periodically transmitted from the central antenna array36. The phase relation between the modulated signal and the referencesignal at the aircraft will be dependent on the location of the usingaircraft within the cardioid pattern.

To improve the accuracy of bearing information transmitted from thecentral antenna array 36, a group of nine split parasitic elements 100are mounted to rotate with the parasitic elements 98 about the dipoles82 and 84. These nine parasitic elements rotate around the dipoles 82and 84 with the parasitic elements 98 and modify the cardioid radiationpattern, also shown in FIG. 5. Although the basic cardioid pattern isstill dominant it is altered by superimposed ripples. Using nineparasitic elements the maxima of these ripples, or minor lobes, arespaced 40 apart. To furnish a suitable reference for measuring the phaseof the high frequency component of the envelope wave a coded highfrequency reference burst similar to the low frequency reference burstis sequentially transmitted from the central antenna array 36.

In one model of an antenna the drive motor 44 rotated the parasiticelements 98 and 100 at 900 revolutions per minute. This produced a lowfrequency amplitude modulation component of Hz and a high frequencymodulation component superimposed of the low frequency of 135 Hz.

In order to further improve the uptilt of the radiation pattern, each ofthe parasitic elements 98 and 100 is made in two parts such that theupper part will have an impedance to cause current flow to be retardedwith respect to current flow in the lower part when both aresimultaneously excited. Thus, in FIG. 3, each of the parasitic elements98 and 100 is shown as made up of an upper part and a lower part.

The technique used for generating the low frequency and high frequencyreference trigger burst uses an infrared detecting device which whenoperated in conjunction with a light interrupting disc 102 createstiming pulses which are synchronized with the rotation of the parasiticelements 98 and 100. Three timing signals are generated; one conveys thephase of the low frequency modulating component, a second conveys thephase of the high frequency modulating component and the third the tenthharmonic of the high frequency component which serves to generate theperiodically generated identification pulse train.

Referring to FIG. 3, the three timing pulses are generated by employinglight emitting diodes 104, 106 and 108 above the light interrupting disc102. Light passing from the emitting diodes through the rotating disc102 impinges on photosensitive transistors 110, 112 and 1 14 as part ofthree trigger amplifiers. In the configuration shown, each of thetransistors has an emitter electrode connected to ground through a line116 which in turn connects to the reference trigger amplifier 64.Individual collector electrodes of the transistors are also connected toinputs of the'reference trigger amplifier 64. The amplifier 64 servingto increase the signal level for transmission to the control/transferunit 16.

The rotating light interrupting disc 102 is a metal disc withtransparent slots. This disc is schematically illustrated in FIG. 5superimposed with the cardioid radiating pattern from the centralantenna array. Slots are opened on three diameters with the single slot118 providing the low frequency timing pulses. The intermediate diameterslots provide the high frequency modulation component and inner diameterslots provide the tenth harmonic of the high frequency component foridentity purposes.

In the model of the antenna previously referred to wherein the lowfrequency component was 15 Hz and the high frequency component was 135Hz, the outermost diameter of the disc 102 generates a 15 Hz timingsignal for the low frequency reference signal, the middle diametergenerates the 135 Hz timing signal for the high frequency referencepulses and the innermost diameter generates a 1,350 Hz timing signal foridentification. With nine parasitic elements 100, each are displaced 40and the slots on the middle diameter also have a 40 separation. Theinner diameter slots have a 4 separation.

In operation of the trigger sensor, a trigger pulse is generated by theapplicable photosensitive transistor each time a slot passes between thelight source and the transistor. A bracket holding the light sources andthe photosensitive transistors is adjustable to obtain proper referencealignment between the rotating parasitic elements 98 and and thereference pulses.

Also located within the antenna 10 and connected to the control unit 14are motor power factor correcting capacitors 120, 122 and 124. Each ofthese capacitors is located across two of the three phase input lines ofthe motor 44. They compensate for the inductive load of the motor suchthat drive circuitry of the antenna control 14 sees an essentiallyresistive type load. Typically, the motor 44 may be a three phasesynchronous motor to provide the required timing operation.

Referring to FIGS. 6 and 7, there is shown in crosssection oneembodiment of an antenna in accordance with the present invention. Thecentral antenna array 38 and the parasitic elements 98 and 100 areclosed within a radome 126 comprising an upper half 126a fastened to alower half 126b to form a weather tight enclosure. This enclosure isbolted to a mounting plate 128 of a pedestal that forms a housingsupport. The housing support also includes spacer bars 130 bolted to alower mounting plate 132. A shroud 134 encloses the structure whichcontains the drive motor 44 and feed cable connections to the centralantenna array 36. With reference to FIG. 1, the lower mounting plate 132is supported on the tripod 12.

In the embodiment of the antenna shown in FIGS. 6 and 7, all metalsupport parts are within a diameter no greater than the diameter of theshroud 134. By restricting the use of metallic parts to the smallestdiameter possible excitation of parts having spillover characteristicswhich illuminate regions on the underside and also at high angles on thetopside is eliminated. Particular effort was made to keep all parts ofthe rotating cylinder 154 of nonmetallic material except at points ofattachment to the drive motor 44. The same holds true for materials-ofthe radome 126.

With emphasis on FIG. 7, the motor housing 136 is attached to the lowermounting plate l32'and includes a motor shaft 138 secured to the frame136 and fixed in position relative to the housing. At the upper andlower end of the shaft 138 are positioned bearings 140 and 142. Mountedto these bearings is a rotor tube 144 which rotates with respect to theshaft 138 and the antenna housing. At the lower end of the rotor tube144 there is press fit and affixed by means of an adhesive a rotorsleeve 146 which functions as the armature of a conventional three phasesynchronous motor. A field coil 148 is included within the motor housing136 and is wound on a stater 150 to provide the rotating magnetic fieldto the rotor sleeve 146. The impedance of the motor structure to theradiated energy from the central array 36 provides additional uptilt ofthe radiation pattern asshown in FIG. 4.

The upper part of the rotor tube 144 includes a flange 152 that supportsa rotating drum 154. Preferably, the rotating drum 154 is of a thindielectric material, such as fiberglass. It is a spiral wound support onwhich there is mounted the nine split parasitic elements 100 on the thinwalls thereof. As explained previously, the parasitic elements 100 areof two parts with the upper part longer than the lower. Typically, theparasitic elements 100 may be a wire having a resistance of 51 ohms perfoot with the upper part slightly longer than a half wavelength of themid-band operating frequency and the lower part slightly less than ahalf wavelength of the operating frequency. The nine wires are spacedaround the drum at 40 intervals and, in the embodiment shown, areadhered to the outside surface with an adhesive.

Also rotating with the drum 154 is a support tube 156 that is attachedto the flange 152 by means of a collar 158. The support tube 156 is alsoof a thin dielectric material, such as fiberglass, and supports thesplit parasitic elements 98 on spaced filaments 159 extended betweencollars 160. To maintain a spaced relationship between the drum 154 andthe tube 156, the upper end of the tube is fitted with a positioningring 162 fastened to the upper wall of the drum 154.

The central antenna array 36 connects to the coaxial cable 50 through aconnector 164 and a coaxial line 166 including a main transmission line166a and a center conductor 16617. Positioned along the line 166 areradio frequency chokes 168 and 171) which function to add to the uptiltof the radiation pattern from the antenna. Each of the chokes 168 and170 are one-quarter wavelength long at the mean operating frequency andare spaced apart along the line 166 a quarter wavelength and have ageneral outline in the form of a cylinder with one end closed. Thechokes 168 and 170 are spaced from the central array 36 such that energytransmitted from the central array is in phase with the intended signalfrom the radiation feed point 167.

Within the rotating drum 154, the central antenna array 36 is positionedin a low loss antenna tube 172 fastened to the motor shaft 138. Asecondary transmission line or distribution transmission line 174 isarranged around the main transmission line. This secondary transmissionline is provided with radiation skirts 176, 178, 180 and 182. Skirts 176and 182 have a configuration of a cylinder with one end closed, seeskirt 176, the skirt 182 opening downward. Energy from a main feed point167 of the antenna is fed into the secondary transmission line 174anddivided in equal parts in both directions along the main transmissionline 166a. Equal portions of the energy from themain feed point arriveat the half wave radiator points 184 and 186 with the skirts 176, 178,181) and 182 radiating the energy from the antenna. Each of the skirtsis substantially one-quarter wavelength long. Energy from the radiatorpoints 184 and 186 is radiated by the skirts 176, 178 180 and 182 witheach skirt being arranged along the transmission line 174 at one-quarterwavelength intervals. Skirts 176 and 182 are separated from thetransmission line 174 such that energy beyond the onequarter wavelengthskirt circulates through the skirt region thus providing 180 of phaseshift and thereby contributing to a cancellation of RF leakage beyondthat point. Thus, the net radiating area provides half wave radiators.

At the top of the main transmission line 166a, there is positioned anenergy absorbing cavity 188 filled with an absorbing material, such aspolyurethane foam. The cavity 188 is formed within the tube 172 by discs190 and 192.

As constructed, the central antenna array 36 is two stacked dipoleelements having a vertically polarized circular radiation pattern. Asthe radiated energy leaves the central array it passes through thecylindrical tube 172 and illuminates the tube parasitic elements 98which provide a low frequency amplitude modulation of the radiatedenergy. The radiated energy continues through the cylindrical tube 156to the periphery of the fiberglass rotating drum 154 where the nineparasitic elements are illuminated. These elements act to furthermodulate the radiated energy to superimpose a high frequency componenton the low frequency component provided by the parasitic elements 98.

For symmetry purposes to provide equal current into the elements of thecentral antenna array, a section of the main transmission line 166aextends above the center conductor 16617. The line 166a has leakagecurrents which are phased by the tube length. Thus, the total carrierradiation system extends from the top of the main transmission line 166adown to a point in the rotating mechanism of the motor 44. This lengthis considerably greater than the length of the antenna elements in thecentral array. Although these extreme portions carry very smallcurrents, they are effective in modifying the radiation at right anglesto the system to further improve the direct radiation pattern, that is,to add to the uptilt of the radiation pattern above the horizon.

The nonmetallic underside of the rotating drum 152 and the radome 128also prevents RF spillover, thereby reducing the energy radiated belowthe horizon to further improve the percentage of direct radiation abovethe horizon from the system.

Referring to FIG. 8, there is shown an exploded view of the antennasystem including the upper part 126a of the radome 126 and the lowerpart 126b which attaches to the support mounting plate 128. Above themounting plate 128, as also shown in FIG. 7, there is located thereference trigger sensor 48. The light disc 102 is attached to the rotortube 144 for rotation therewith.

In assembly of the antenna, the support tube 172 with all the elementsof the central antenna array 36 included therein is attached to thenonrotating motor shaft 138 at the center top of the flange 152. Thechokes 168 and and the feed line 166 pass through the motor shaft 138.This assembly along with the radome 126 are fixed in position withrespect to the antenna housing support. Theparasitic support tube 156with the parasitic elements 98 assembled thereto is fastened to therotating drum 154 with the complete assembly mounted to the flange 152for rotation with respect to the central antenna array 36. The referencetrigger sensor 48 remains stationary in the housing and contains thephotodiodes 104, 106 and 108 and the phototransistors l 10, 112 and 1 14with the disc 102 rotating with the rotating drum 154.

Referring to FIG. 9, there is shown an alternate embodiment for mountingthe parasitic elements 100 to the rotating drum 1S4. Spacers 198 areattached to the thin walls of the drum at each location of an element100. Between the spacers 198 there is stretched a filament 200 which maybe of nylon, mylar or similar material. The two parts of the parasiticelements are then attached to the filament 200 with the longest partadjacent the upper spacer.

An arrangement like that shown in FIG. 9 mounts the elements 100 inessentially the same manner as shown in FIG. 7 for the elements 98. Afilament mounting away from the surface of the rotating drum 154 eitheron the inside or outside of the rotating drum 154 has been found tofurther increase the uptilt pattern of the radiation pattern from thecentral antenna array.

While several embodiments of the invention, together with modificationsthereof, have been described in detail herein and shown in theaccompanying drawings, it will be evident that various furthermodifications are possible without departing from the scope of theinvention.

What is claimed is:

1. An antenna system for obtaining uptilt of radiation above the horizonand energized from a radio frequency source, comprising in combination:

a central antenna array including:

a. a coaxial transmission line connected to the radio frequency sourceand having a feed point,

b. a first energy radiator coupled to said feed point along thetransmission line to have a current therein with a lagging phaserelationship with ref erence to a current in the feed point, and

c. a second energy radiator coupled to said feed point along thetransmission line to have a current therein with a leading phaserelationship with reference to current in a feed point and said firstradiator, and

at least one radio frequency choke positioned along the transmissionline of said antenna array in a spaced relationship with the secondradiator to produce a leading current with reference to a current in thesecond radiator along said transmission line between the second radiatorand said choke to uptilt the radiation pattern from the antenna system.

2. A radio frequency antenna system as set forth in claim 1 including acavity at the end of said central antenna array filled with an energyabsorbing material.

3. An antenna system for obtaining uptilt of radiation above the horizonand energized from a radio frequency source, comprising in combination:

a support including a drive motor,

a central antenna array mounted to the support and including:

a. a feed point connected to the radio frequency source,

b. a first energy radiator coupled to said feed point to have a currenttherein with a lagging phase relationship with reference to a current inthe feed point, and

c. a second energy radiator coupled to said feed point to have a currenttherein with a leading phase relationship with reference to current inthe feed point and said first radiator,

a plurality of spaced lagging current reflectors positioned radiallyfrom the first radiator and responsive to energy therefrom to generate acurrent therein with a lagging phase relationship with reference to acurrent in said first radiator and said feed point,

a plurality of spaced leading current reflectors positioned in alignmentwith the lagging current reflectors and radially from the secondradiator and responsive to energy therefrom to generate a currenttherein with a leading phase relationship with reference to the currentin said second radiator and said feed point, and

a support drum of a dielectric material with one end coupled to thedrive motor and the lagging and leading current reflectors attached tothe wall of said drum.

4. A radio frequency antenna system as set forth in claim 3 wherein saiddrum of dielectric material further includes spacers mounted to saiddrum with a filament stretched between pairs of said spacers and theleading and lagging current reflectors attached to said filaments.

5. An antenna system as set forth in claim 3 wherein said drum has adiameter substantially larger than a maximum dimension of said supportto minimize radiation reflected from said support to said spaced currentreflectors.

6. An antenna system as set forth in claim 3 including a coaxialtransmission line with the radiating feed point as a part thereof andsaid radiators positioned along the transmission line.

7. An antenna system as set forth in claim 6 including at least oneradio frequency choke coupled to the transmission line of said antennaarray in a spaced relationship with the second radiator to produce aleading current with reference to a current in the second radiator alongsaid transmission line between the second radiator and said choke.

8'. An antenna system for obtaining uptilt of radiation above thehorizon and energized from a radio frequency source, comprising incombination:

a central antenna array including:

a. a feed point connected to the radio frequency source,

b. a first energy radiator coupled to said feed point to have a currenttherein with a lagging phase re lationship with reference to a currentin the feed point, and

c. a second energy radiator coupled to said feed point to have a currenttherein with a leading phase relationship with reference to current inthe feed point and said first radiator,

a plurality of spaced lagging current reflectors positioned radiallyfrom the first radiator and responsive to energy therefrom to generate acurrent therein with a lagging phase relationship with reference to acurrent in said first radiator and said feed point, and

a plurality of spaced leading current reflectors positioned along thesame radial as the lagging current reflectors and radially from thesecond radiator and responsive to energy therefrom to generate a currenttherein with a leading phase relationship with reference to the currentin said second radiator and said feed point.

9. A radio frequency antenna system as set forth in claim 8 includingsupport means including a drum of a dielectric material with one endcoupled to a drive motor and the leading and lagging current reflectorsattached to the walls of said drum.

10. A radio frequency antenna system as set forth in claim 8 wherein thelagging current reflectors have an impedance to cause current flow to beretarded with respect to current flow in the leading current reflectorswhen both are excited simultaneously.

11. A radio frequency antenna system as set forth in claim 8 wherein thelagging current reflectors are longer than a half wavelength of theelectromagnetic wave radiating from said central antenna array and theleading current reflectors are shorter than a half wavelength.

12. An antenna system as set forth in claim 8 including a coaxialtransmission line with the radiating feed point as a part thereof andsaid radiators positioned along the transmission line.

13. An antenna system as set forth in claim 12 including at least oneradio frequency choke positioned along the transmission line of saidantenna array in a spaced relationship with the second radiator toproduce a leading current with reference to a current in the secondradiator along said transmission line between the second radiator andsaid choke.

14. An antenna system as set forth in claim 8 wherein said first energyradiator and said second energy radiator are half wavelength dipoles.

15. A radio frequency antenna system for obtaining uptilt of radiationabove a reference line, comprising in combination:

a support including a drive motor,

a central antenna array of two stacked omnidirectional dipole radiatorsmounted to said support to be fixed in position with respect theretoalong a coaxial transmission line having a radiating feed pointenergizing one dipole to have a leading current with respect to the feedpoint and the second dipole to have a lagging current with espect to thefeed point to radiate a pattern above a reference line,

at least one radio frequency choke positioned along the transmissionline of said antenna array at the support and in proximity to one ofsaid dipoles to reflect radiation along the transmission line from saidomni-directional antenna to further uptilt the radiation pattern fromsaid central antenna array,-

a plurality of spaced, parasitic reflectors radially psitioned from saidcentral antenna array, and

support means for positioning said plurality of parasiticreflectors andcoupled to the drive motor of said support and rotated thereby toproduce a rotation of said reflectors around the central array formodulating electromagnetic waves radiated therefrom.

16. A radio frequency antenna system as set forth in claim 15 whereinsaid support means includes a drum of a dielectric material with one endcoupled to the drive motor and the parasitic antenna element attached tothe wall of said drum.

17. A radio frequency antenna system as set forth in claim 15 whereinsaid support means includes a drum of a dielectric material with one endcoupled to the drive motor and further including spacers mounted to thewalls of said drum with a filament stretched between pairs of saidspacers and the reflectors attached to said filaments.

18. A radio frequency antenna system for obtaining uptilt of radiationabove a reference line, comprising in combination:

a support including a drive motor,

a central antenna array of omni-directional dipole radiators mounted tosaid support to be fixed in position with respect thereto, one of saidradiators having a lagging current with reference to a feed point andthe second radiator having a leading current with reference to the feedpoint to radiate a pattern above the reference line,

a first plurality of two-part, spaced, parasitic reflectors radiallypositioned from said central antenna array, one part of each of saidreflectors having a length greater than the second part with the longerof the two positioned radially from the first of said radiators andhaving a lagging current with reference to a current in said firstradiator and said feed point and the second part positioned radiallyfrom the second of said radiators and having a leading current withreference to a current in said second radiator and said feed point tofurther uptilt the radiation pattern from said central array,

a second plurality of spaced parasitic reflectors radially positionedbetween said central antenna array and said first plurality of parasiticreflectors,

first support means for positioning said first plurality of parasiticreflectors and coupled to the drive motor of said support and rotatedthereby to provide rotation of said reflectors around the central arrayfor modulating the electromagnetic waves radiated therefrom at a firstfrequency, and

second support means for positioning said second plurality of parasiticreflectors and coupled to the drive motor of said support and rotatedthereby to provide rotation of said reflectors around the central arrayfor modulating electromagnetic waves radiated therefrom at a secondmodulation frequency.

19. A radio frequency antenna system as set forth in claim 18 whereineach of said second plurality of spaced parasitic reflectors includes afirst part having a length greater than the second part with the longerof the two parts positioned above the reference line to uptilt theradiation pattern from said central antenna array.

20. A radio frequency antenna system as set forth in claim 18 whereinsaid first support means includes a drum of a dielectric material withone end coupled to the drive motor and the parasitic reflectors-attachedto the walls of said drum.

21. A radio frequency antenna system as set forth in claim 18 whereinsaid first support means includes a drum of a dielectric material withone end coupled to the drive motor and further including spacers mountedto the walls of said drum with a filament stretched between pairs ofspacers and the two parts of each parasitic reflector attached to saidfilaments.

22. A radio frequency antenna system as set forth in claim 18 includingat least one radio frequency choke coupled to the omni-directionalantennas of said central array to provide a main lobe beam angle abovethe reference line.

1. An antenna system for obtaining uptilt of radiation above the horizonand energized from a radio frequency source, comprising in combination:a central antenna array including: a. a coaxial transmission lineconnected to the radio frequency source and having a feed point, b. afirst energy radiator coupled to said feed point along the transmissionline to have a current therein with a lagging phase relationship withreference to a current in the feed point, and c. a second energyradiator coupled to said feed point along the transmission line to havea current therein with a leading phase relationship with reference tocurrent in a feed point and said first radiator, and at least one radiofrequency choke positioned along the transmission line of said antennaarray in a spaced relationship with the second radiator to produce aleading current with reference to a current in the second radiator alongsaid transmission line between the second radiator and said choke touptilt the radiation pattern from the antenna system.
 2. A radiofrequency antenna system as set forth in claim 1 including a cavity atthe end of said central antenna array filled with an energy absorbingmaterial.
 3. An antenna system for obtaining uptilt of radiation abovethe horizon and energized from a radio frequency source, comprising incombination: a support including a drive motoR, a central antenna arraymounted to the support and including: a. a feed point connected to theradio frequency source, b. a first energy radiator coupled to said feedpoint to have a current therein with a lagging phase relationship withreference to a current in the feed point, and c. a second energyradiator coupled to said feed point to have a current therein with aleading phase relationship with reference to current in the feed pointand said first radiator, a plurality of spaced lagging currentreflectors positioned radially from the first radiator and responsive toenergy therefrom to generate a current therein with a lagging phaserelationship with reference to a current in said first radiator and saidfeed point, a plurality of spaced leading current reflectors positionedin alignment with the lagging current reflectors and radially from thesecond radiator and responsive to energy therefrom to generate a currenttherein with a leading phase relationship with reference to the currentin said second radiator and said feed point, and a support drum of adielectric material with one end coupled to the drive motor and thelagging and leading current reflectors attached to the wall of saiddrum.
 4. A radio frequency antenna system as set forth in claim 3wherein said drum of dielectric material further includes spacersmounted to said drum with a filament stretched between pairs of saidspacers and the leading and lagging current reflectors attached to saidfilaments.
 5. An antenna system as set forth in claim 3 wherein saiddrum has a diameter substantially larger than a maximum dimension ofsaid support to minimize radiation reflected from said support to saidspaced current reflectors.
 6. An antenna system as set forth in claim 3including a coaxial transmission line with the radiating feed point as apart thereof and said radiators positioned along the transmission line.7. An antenna system as set forth in claim 6 including at least oneradio frequency choke coupled to the transmission line of said antennaarray in a spaced relationship with the second radiator to produce aleading current with reference to a current in the second radiator alongsaid transmission line between the second radiator and said choke.
 8. Anantenna system for obtaining uptilt of radiation above the horizon andenergized from a radio frequency source, comprising in combination: acentral antenna array including: a. a feed point connected to the radiofrequency source, b. a first energy radiator coupled to said feed pointto have a current therein with a lagging phase relationship withreference to a current in the feed point, and c. a second energyradiator coupled to said feed point to have a current therein with aleading phase relationship with reference to current in the feed pointand said first radiator, a plurality of spaced lagging currentreflectors positioned radially from the first radiator and responsive toenergy therefrom to generate a current therein with a lagging phaserelationship with reference to a current in said first radiator and saidfeed point, and a plurality of spaced leading current reflectorspositioned along the same radial as the lagging current reflectors andradially from the second radiator and responsive to energy therefrom togenerate a current therein with a leading phase relationship withreference to the current in said second radiator and said feed point. 9.A radio frequency antenna system as set forth in claim 8 includingsupport means including a drum of a dielectric material with one endcoupled to a drive motor and the leading and lagging current reflectorsattached to the walls of said drum.
 10. A radio frequency antenna systemas set forth in claim 8 wherein the lagging current reflectors have animpedance to cause current flow to be retarded with respect to currentflow in the leading current reflectors when both are excitedsimultaneously.
 11. A radio Frequency antenna system as set forth inclaim 8 wherein the lagging current reflectors are longer than a halfwavelength of the electromagnetic wave radiating from said centralantenna array and the leading current reflectors are shorter than a halfwavelength.
 12. An antenna system as set forth in claim 8 including acoaxial transmission line with the radiating feed point as a partthereof and said radiators positioned along the transmission line. 13.An antenna system as set forth in claim 12 including at least one radiofrequency choke positioned along the transmission line of said antennaarray in a spaced relationship with the second radiator to produce aleading current with reference to a current in the second radiator alongsaid transmission line between the second radiator and said choke. 14.An antenna system as set forth in claim 8 wherein said first energyradiator and said second energy radiator are half wavelength dipoles.15. A radio frequency antenna system for obtaining uptilt of radiationabove a reference line, comprising in combination: a support including adrive motor, a central antenna array of two stacked omni-directionaldipole radiators mounted to said support to be fixed in position withrespect thereto along a coaxial transmission line having a radiatingfeed point energizing one dipole to have a leading current with respectto the feed point and the second dipole to have a lagging current withespect to the feed point to radiate a pattern above a reference line, atleast one radio frequency choke positioned along the transmission lineof said antenna array at the support and in proximity to one of saiddipoles to reflect radiation along the transmission line from saidomni-directional antenna to further uptilt the radiation pattern fromsaid central antenna array, a plurality of spaced, parasitic reflectorsradially positioned from said central antenna array, and support meansfor positioning said plurality of parasitic reflectors and coupled tothe drive motor of said support and rotated thereby to produce arotation of said reflectors around the central array for modulatingelectromagnetic waves radiated therefrom.
 16. A radio frequency antennasystem as set forth in claim 15 wherein said support means includes adrum of a dielectric material with one end coupled to the drive motorand the parasitic antenna element attached to the wall of said drum. 17.A radio frequency antenna system as set forth in claim 15 wherein saidsupport means includes a drum of a dielectric material with one endcoupled to the drive motor and further including spacers mounted to thewalls of said drum with a filament stretched between pairs of saidspacers and the reflectors attached to said filaments.
 18. A radiofrequency antenna system for obtaining uptilt of radiation above areference line, comprising in combination: a support including a drivemotor, a central antenna array of omni-directional dipole radiatorsmounted to said support to be fixed in position with respect thereto,one of said radiators having a lagging current with reference to a feedpoint and the second radiator having a leading current with reference tothe feed point to radiate a pattern above the reference line, a firstplurality of two-part, spaced, parasitic reflectors radially positionedfrom said central antenna array, one part of each of said reflectorshaving a length greater than the second part with the longer of the twopositioned radially from the first of said radiators and having alagging current with reference to a current in said first radiator andsaid feed point and the second part positioned radially from the secondof said radiators and having a leading current with reference to acurrent in said second radiator and said feed point to further uptiltthe radiation pattern from said central array, a second plurality ofspaced parasitic reflectors radially positioned between said centralantenna array aNd said first plurality of parasitic reflectors, firstsupport means for positioning said first plurality of parasiticreflectors and coupled to the drive motor of said support and rotatedthereby to provide rotation of said reflectors around the central arrayfor modulating the electromagnetic waves radiated therefrom at a firstfrequency, and second support means for positioning said secondplurality of parasitic reflectors and coupled to the drive motor of saidsupport and rotated thereby to provide rotation of said reflectorsaround the central array for modulating electromagnetic waves radiatedtherefrom at a second modulation frequency.
 19. A radio frequencyantenna system as set forth in claim 18 wherein each of said secondplurality of spaced parasitic reflectors includes a first part having alength greater than the second part with the longer of the two partspositioned above the reference line to uptilt the radiation pattern fromsaid central antenna array.
 20. A radio frequency antenna system as setforth in claim 18 wherein said first support means includes a drum of adielectric material with one end coupled to the drive motor and theparasitic reflectors attached to the walls of said drum.
 21. A radiofrequency antenna system as set forth in claim 18 wherein said firstsupport means includes a drum of a dielectric material with one endcoupled to the drive motor and further including spacers mounted to thewalls of said drum with a filament stretched between pairs of spacersand the two parts of each parasitic reflector attached to saidfilaments.
 22. A radio frequency antenna system as set forth in claim 18including at least one radio frequency choke coupled to theomni-directional antennas of said central array to provide a main lobebeam angle above the reference line.